This invention relates to photographic objectives, are more particularly to the correction of an illumination distribution on the image plane against deterioration when the diaphragm stops down.
Interchangeable photographic objectives having standard or semi-standard angles of view, for use in single lens reflex cameras, employ an arrangement in which the diaphragm is located at or near the center of the physical length of the objective. However, lens shutter-equipped cameras which require further minimization in the bulk and size of their objective lenses have made increasing use of (1) a diaphragm behind the entire lens system, i.e., a so-called rear diaphragm lens, or (2) a lens with the diaphragm followed by a component of a negative power.
As these types (1) and (2) of objectives have a large angular magnification of the principle ray of light of the pupil, when the diaphragm stops down, the uniformity of illumination distribution on the image plane tends to deteriorate at the corners of the film frame more sharply than the other types of objectives, for example, objectives with the diaphragm followed by a positive component.
The rear diaphragm lens and negative component-follow-stop type objectives have the advantage that when fully open, the marginal illumination can be made rather higher than when the lens component behind the stop is positive. When the lens is closed down by two or three stops from the fully open position, however, the ratio of the marginal illumination to the central illumination becomes larger than when the lens is of the positive component-follow-stop type.
Standard photographic lenses regardless of the lens construction, are easy to design even for the oblique pencil from the maximum angle of view to pass through almost the entire area of the aperture usually when stopped down by two or three steps. With such design, when applied to the positive component-follow-stop type, the corner illumination is not less than about 90% of the central illumination at a maximum semi-angle view of, for example, 27.degree., but when applied to the behind stop type or the negative component-follow-stop type, it is very difficult to reach 70% at least. As a result, the illumination distribution over the entire area of the film frame has been left with as low a uniformity as 40%.